From earliest times the need has existed for effective anchoring systems for vessels to resist wind and current. Single anchors have given way to multiple anchors, sea anchors and a variety of anchor handling techniques to precisely position and securely hold a vessel against wind and current.
A whole new dimension in anchoring and anchor handling arose with the expansion of offshore drilling which employs a floating drilling barge that needs to be located and maintained over an oil well located on the ocean bottom. Due to the immense cost of offshore drilling operations, the continuation of these drilling operations during adverse weather conditions, even with up to fifteen foot waves, is essential to economically proceed with such operations.
Similarly, submarine pipeline laying operations require the precise movement of a pipeline laying vessel along a specific course. The submarine pipeline laying operation is preferably continuous since interruption of the operation presents even greater difficulties upon resumption than is the case for the offshore drilling operation.
During normal drilling operations a number of anchoring systems have evolved for positioning the drilling vessel, e.g. barge, by employing from eight (8) to as many as fourteen (14) anchors. One essential element for this anchoring system is an automatic positioning system that simultaneously controls all anchor lines. One example of an improved pipeline laying barge is described in the article The Third Generation Lay Barge by G. H. G. Lagers et al. copyright 1974, Offshore Technology Conference design parameters for improved stability for a pipeline laying barge or a moored drilling vessel by employing dynamic controls are described in the article Augmentation of a Mooring System Through Dynamic Positioning by J. S. Sargent et al, copyright 1974, Offshore Technology Conference. Both articles was presented at the Sixth Annual Offshore Technology conference at Houston, Texas May 6-8, 1974.
The dynamics of deep water anchoring systems and a fundamental block diagram for manual or automatic feedback control systems for mooring lines either alone or in combination with thrusters is described in an article by Alan C. McClure, Naval Architect, that appears on pages 18-24 of the Feb 1977 of Ocean Resources Enginering.
Finally, a number of patents have issued on automated ship control systems and mooring aids. These patents include:
______________________________________ A. BOUY MOORING SYSTEMS 3,980,038 Dashew et al 9/14/76 3,956,742 R. D. Karl 5/11/76 B. ALONG SIDE MOORING 3,965,841 H. M. W. Croese 6/29/76 4,055,137 Motai et al 10/25/77 3,913,396 G. Elliot 10/25/75 3,886,887 Cunningham et al 6/03/75 3,613,625 Halsingborg et al 10/19/71 C. MULTIPLE ANCHOR MOORING Re 29,373 H. C. Boschen Jr. 8/30/77 3,948,201 I. Takeda et al 4/06/76 4,070,981 Guinn et al 1/31/78 3,552,343 P. Moulin 1/21/69 3,031,997 W. A. Nesbitt 5/01/62 D. SUBMARINE PIPELINE LAYING 3,893,404 Chandler et al 7/08/75 E. SUBMERGED CABLE ADVANCED VESSEL 3,785,326 S. B. Mullerheim 1/05/77 F. SONAR POSITION SENSING SYSTEM 4,017,823 Cooke et al 4/12/77 ______________________________________
In each of the above-referenced systems, cable payout information, if essential to control, is obtained only indirectly by sensors coupled to winches or idle rollers. However, sensors coupled to winches or idler rollers sensors tend to produce a certain amount of errors due to the cable slippage that is typical in such systems Similarly, the payout or reel-in speed of the cable, which are important in large maneuvering and where there are two corresponding anchors that are preferably synchronously moved, will be incorrectly measured as a result of this cable slippage. One means for eliminating the effect of slippage, is to directly couple the sensors to cable drums. However, such systems have been unable to account for the unevenness of cable layerings, and the changing of cable length due to layer change and therefore only provide average or approximate values.